Drive arrangement for variable width z-fold chart paper

ABSTRACT

A drive arrangement for Z-fold chart paper is provided in which spaced apart guide means positioned between the back tensioning roller and the chart supply provide a positive lateral reference for the chart web just prior to its engagement with the back tensioning roller. The guide means may be conveniently changed in spacing to accommodate chart paper of different widths without requiring any readjustment of the drive rollers or the back tensioning roller.

United States Patent Erbach et al. 1 Feb. 29 1972 [54] DRIVE ARRANGEMENTFOR 2,973,081 2/1961 Maidment et al ..197/l33 F VARIABLE WIDTH Z-F()LDCHART 3,364,498 1/1968 Kawase et a1. ..346/136 PAPER 3,552,621 1/1971Kupferschmidt ..226/174 [72] Inventors: Arthur R. Erbach, Des Plaines;Albert FORElGN PATENTS 0R APPLICATIONS Kupfemfimld'i chlcago 690,2114/1953 Great Britain ..197/133 F [73] Assignee: Beckman Instruments,Inc.

Primary Examiner-Joseph W. Hartary Flled! P 13, 1970 Attorney-Mason,Kolehmainen, Rathbum 8:. Wyss [21] Appl. No.: 27,473 ABSTRACT [52] U SCl 346/136 226,199 A drive arrangement for Z-fold chart paper isprovided in [51] In} .0 G6. 15,30 which spaced apart guide meanspositioned between the back I 58] Fieid I198 tensioning roller and thechart supply provide a positive lateral 33 270/6l 52 reference for thechart web just prior to its engagement with the back tensioning roller.The guide means may be conveniently changed in spacing to accommodatechart paper of [56] 3 Cited different widths without requiring anyreadjustment of the UNITED STATES PATENTS drive rollers or the backtensioning roller 2,077,428 4/1937 Mabon ..197/1 33 F 9 Claims, 4Drawing Figures PAIENTEnFEB29 I972 3. 646.569

sum 1 [IF 3 FIG. 1

DRIVE ARRANGEMENT FOR VARIABLE WIDTH Z- FOLD CIIART PAPER The presentinvention relates to direct writing oscillograph recorders, and, moreparticularly, to a drive arrangement for such recorders which is adaptedto accommodate different widths of Z-fold chart paper.

Some recorder chart transport mechanisms utilize drive roller meanswhich are constructed to correct for minor lateral motions of the movingchart, thus providing a nonskew, or so-called zero-weave motion of thechart. While these arrangements work satisfactorily when the chart is ina roll form, which can be positively located on a shaft and orientedproperly to the drive path, these mechanisms are generally lesseffective in controlling skew, or weave, when the chart is in the formof a flat package of Z-fold sheets. Nonskew drives usually are arrangedto correct the motion of the chart by minute slippage of one or severalroller sections. This slippage occurs when lateral motion of the chartcreates increased tension on one edge of the chart and decreased tensionon the opposite edge. The drive roller section or sections experiencingthe increased load will slip, or stall, momentarily, allowing therollers on the slack side to balance the edge tensions, after whichnormal drive is resumed.

Since all normal drive conditions for such nonskew or zeroweave drivemechanisms are based on some reference position of the chart, fulleffectiveness of a nonskew drive requires a constant reference locationfor the chart edges. When Z-fold paper begins to unfold from thebasepack to travel through the transport mechanism, it is extremelycompliant and unstable in the lateral direction. In most chart drivesdesigned for Z- fold chart paper, edge guides are placed at the formingedge, or lead edge of the writing table in an effort to orient the chartimmediately before it passes beneath the recording pen. This form ofguiding the edges of the chart is reasonably satisfactory if nosignificant back-tension is maintained in a chart web passing throughthe transport mechanism. However, in order to reduce distortions of therecord at the folds of Z-fold chart paper, it is necessary to greatlyincrease the back-tension of the chart web and this increasedback-tension causes any nonparallelism of guide members to producelateral forces on the chart which will cause skewing of the chart unlessit is controlled by a nonskew drive system. However, the forming edgeguides associated with the writing table do not provide a satisfactoryreference position for the chart and thus the nonskew drive is notfully'effective. The addition of a locating wall against which the chartpackage isregistered does bring the reference condition up to a usablelevel. The drive will then correct skewing, but the result is usually analternate weaving, or hunting, about some mean point and the amplitudeof weave is objectionably high, often up to as high as percent of therecording channel width on the paper.

- In many instances it is also economically desirable to recordinformation using a chart paper of a width which is less than themaximum chart width that the chart transport will drive. If, forexample, only 3 parameters are being recorded, it is more economical touse four-channel chart paper than the higher cost eight-channel paper,even though the recorder will accept the eight channel chart. Twodifficulties arise in most recorder transport mechanisms under theseconditions- Either the chart must be centered in a nonskew drive system,with the attendant mechanical adjustment of drive rollers, pressurerollers and guides, or in the alternative a simple edge drive must beused, thus eliminating the advantages of a zero-weave drive arrangement.In either case, there is usually some adjustment necessary for theback-tension required with charts of different widths. These variousadjustments are often so time consuming as to offset the difference inthe cost of the chart paper used.

It is, therefore, an object of the present invention to provide a newand improved Z-fold chart paper drive arrangement which eliminates oneor more of the above discussed disadvantages of prior art arrangements.

It is a further object of the present invention to provide a new andimproved Z-fold chart paper drive arrangement a which is arrangedtodrive chart paper of various widths with a minimum number of adjustmentsrequired to adapt the drive mechanism to different widths of chartpaper.

It is another object of the present invention to provide a new andimproved chart paper drive arrangement which is adapted to function witha Z-fold chart paper pack and wherein spaced apart guide means arepositioned between the back tensioning roller and the chart pack toprovide a positive lateral reference for the chart web prior to itsengagement with the back tensioning roller.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawings,in which:

FIG. 1 is a perspective view of a direct writing oscillograph recorderembodying the features of the present invention;

FIG. 2 is a sectional view taken along the lines 22 of FIG. 1 andshowing the chart drive mechanism in recording position with a pack ofZ-fold chart paper inserted and threaded through its path to the driverollers of the recorder;

FIG. 3 is a sectional view taken along the lines 3-3 of FIG.

2 and illustrating the edge guide arrangement of the present invention;and

FIG. 4 is a sectional view taken along the lines 44 of FIG. 2.

Referring now to the drawings, the horizontal table chart drivemechanism of the recorder is shown installed within a floor type consoleenclosure, indicated generally at 10, and cooperates with a plurality ofrecorder pens 12 which are driven by means of individual pen motors 14in response to electrical input signals whichare to be recorded on thecommon chart 16 of the recorder. In the illustrated embodiment, thechart 16 comprises a pack of eight-channel Z-fold or fanfold chartpaper, which is positioned at a reference location on ashelf 18 withinthe console cabinet 20 of the recorder and is accessible through a door22 provided in the end of the cabinet 20. The'active chart web 24, ofthe chart pack 16 extends upwardly through the various rollers and guidebars of the chart drive mechanism to be described in more detail ries ofspaced drive rollers 28 and mating pressure rollers 30.

The drive rollers 28 are preferably of cartridge-type construction, asdescribed in more detail in a copending application of AlbertKupferschmidt, Ser. No. 757,016 filed Sept. 3, 1968, now US. Pat. No3,552,621 which is assigned to the same assignee as the presentinvention. As described in more detail in said copending application,these rollers are arranged to be driven from an electric motor 32 andare designed individually to slip at a predetermined, adjustable maximumtorque so that the Z-fold chart is pulled over the writing table 26against the force of a back tensioning member, to be described in moredetail hereinafter, in a nonskew or so-called zero-weave manner. Thechart is moved at a selected speed so that the recording pens 12 maytrace-out appropriate lines on the chart '16 corresponding to theapplied input signals, the free end of the chart moving over an exitramp 34, and then over the frontwall 36 of the cabinet 20. Moreparticularly, the motor 32 is mounted on one of the side support members31 of the chart transport and is connected to an eight-speed gear boxwhich is electrically controlled by means of a series of push buttons 42mounted in the top wall of the console 10. The output shaft 44 of, thegear box 40 is connected by means of the intermediate gearing to thedrive roller shaft 48 on which the drive rollers 28 are mounted. Theside support members 31 and 33 are connected together through a numberof side bars 35 to form the main support structure of the charttransport mechanism.

Referring now more particularly to the arrangement provided for ease inloading and threading the chart pack into the recorder, under normalrecording conditions the writing table 26 is held in a horizontalposition by means of an overcenter toggle mechanism comprising a tablecoupler link 50 and a drive link 52. More particularly, the writingtable 26 comprises a writing plate or platen 54 and a pair of sidemembers 56 which are pivotally mounted to the side support members 31,33 of the chart transport mechanism by means of the pins 58. A forwardcrossbar 60 is provided to give rigidity to the forward end of the plate54 and a forming bar 62 is provided at the rear of the table 26 overwhich the chart web extends. The

upper end of the table couplerlink 50 is pivotally mounted on anadjustable eccentric cam 65 carried on a pin 63 which is supportedbetween the arms 64 of a U-shaped bracket 67 secured to the under sideof the writing plate 54 by means of the screws 66. The drive link 52comprises a pair of side members 68 which are connected to a drive linkshaft 70 which is rotatable mounted in the side support members 31 and33 of the chart transport mechanism and has an actuating crank 71(FIG. 1) connected to the end thereof exteriorly of the side support 33.A pin 72 connects the upper ends of the side members 68 together and thebottom end of the table coupler link 50 is pivotally mounted on a pin 74which extends between the side members 68.

It will be noted that when the pin 72 engages the edge of the tablecoupler link 50, as shown in FIG. 2, the pivot pin 74 at the bottom endof the link 50 is positioned beyond the centerline 78 connecting thecenters of the drive link shaft 70 and the eccentric cam 65 so that anovercenter toggle action is provided whereby the writing table 26 islocked in a horizontal position. The spring compliance necessary toachieve this overcenter toggle action is provided by flexure of thedrive link shaft 70 due to the fact that the toggle mechanism 50, 52 ismounted approximately midway of the shaft 70, and this shaft may have alength of approximately 16 inches in the case of a recorder usingeight-channel chart paper. The amount of flexure of the shaft 70 andhence the force with which the toggle mechanism 50, 52 is held in lockedposition, may be adjusted by adjustment of the eccentric cam 65. Also,the final position of the writing table 26 and its parallelism to theother guide elements of the system is established by means of a pair ofeccentric earns 80 which are mounted on the side support members 31 ofthe chart transport and engage flat shoulder portions 82 on the sidemembers 56 of the writing table 26.

In order to provide a back tensioning mechanism against which the chartweb may be pulled by the drive rollers 28 so that the chart may bepulled smoothly over the writing table 26 during the recordingoperation, a back tensioning roller 90 is provided which is rotatablymounted on a pair of roller support links 92 the upper end of which ispivotally mounted on the side support members 31, 33 by means of thebearing studs 94. The roller support links 92 are provided with offsetend portions 96 which carry at the free end thereof a guide bar 98 overwhich the chart web passes as it is drawn from the chart pack 16. Thechart web 24 is held against the bar 98 by means of a series of springfingers 102 which are secured to one of the crossbars 35 by means of thescrews 103.

The back-tension roller 90 is provided with an outer surface having avery high coefficient of friction and this roller is constructed so thatan adjustable amount of torque is required to rotate the roller, as willbe readily understood by those skilled in the art. The chart is heldagainst the outer surface of the roller 90 by means of a second guideroll 104 which is mounted between the side support members 31, 33 of thechart transport, and a freely rotating guide roller 106 which is alsorotatably mounted in the side support members 31, 33. The guide roll 104is positioned so as to control the entrance angle of the chart web 24 tothe outer surface of the back-tension roller 90 and the guide roller 106is located so as to hold the chart web 24 against the surface of theroller 90 and to control the point at which the chart leaves thissurface. After the chart web 24 leaves the roller 106, it moves over aguide bar 110, which is supported on the side supports 31 and 33, overthe forming bar 62 of the writing table 26 under the recording pens 12and over the writing plate 54 between the side members 56.

The pressure rollers 30 are rotatably mounted on a shaft 112 which iscarried by a pair of support arms 114 which are pivotally mounted on theside support members 31, 33 by means of the studs 116. The pressurerollers 30 are normally urged against the drive rollers 28 by means of apair of coil springs 118 one end of each of the springs 118 beingconnected to the bottom end of the support members 114 and the other endthereof being connected to the side support members 31, 33 as indicatedat 120. The pressure rollers 30 may be moved away from the drive rollers28 by moving the support members 114 to the loading position in whichposition these members engage the stops 122 on the side supports 31, 33.The chart lead may then be inserted between the rollers 28 and therollers 30, and the rollers 30 moved into engagement with the uppersurface of the chart web so that the drive rollers 28 can pull the chartweb through the chart transport against the drag of the back-tensionroller 90.

Considering now the manner in which the back-tension roller ispositioned in correct relation with respect to the rollers 104 and 106so as to provide the above-described backtension drag on the chart web,a separator bar is connected to and extends between the roller supportlinks 92 and a roller coupler link 132 is employed in conjunction withthe drive link 52 to provide a second overcenter toggle mechanism whichfunctions to lock the back-tension roller 90 and the guide bar 98 intheir respective positions during the recording operation. Moreparticularly, the roller coupler link 132 is rotatably mounted on theseparator bar 130 at the middle thereof and the other end of the link132 is provided with a slot 134 within which is positioned a pin 136which is secured to the side members 68 of the drive link 52.

When the drive link 52 is positioned in the locking position for thewriting table 26, the pin 136 is positioned beyond the centerlineconnecting the centers of the shaft 70 and the separator bar 130 so thatan overcenter toggle mechanism is provided which accurately holds theback-tension roller 90 at the desired position with respect to therollers 104 and 106. The spring compliance necessary to achieve thisovercenter toggle action is provided by the flexure of the separator bar130, and to some extent the flexure of the drive link shaft 70. Thecorrect position of the back tension roller 90 may be adjusted by meansof a pair of eccentric cams which are mounted on the side supportmembers 31, 33 and engage the edges of the roller support links 92.

In accordance with an important aspect of the invention, the guide bars98 and 104 are provided with adjustable guide flanges which constitutespaced-apart guide means positioned between the back tensioning roller90 and the chart pack 16 to provide a positive lateral reference for thechart web prior to its engagement with the back tensioning roller.Furthermore, this guide means is adjustable to accommodate chart paperof different widths without requiring adjustment of the torque settingsof the drive rollers 28. More particularly a pair of guide blocks orcollars 200 and 202 are adjustably positioned on the guide bar 104 bymeans of the thumbscrews 204 and 206, respectively. The portions of theguide blocks 200 and 202 which are on the opposite side of the shaft 104from the thumbscrews 204 and 206 are in close proximity to the backtensioning roller 90 so that the inner edges 206 and 208 of the members200 and 202 act as a first pair of guide flanges to establish a positivelateral reference point for the edges of the Z-fold chart paper 24.

In a similar manner a pair of guide flanges 210 and 212 are adjustablypositioned on the guide bar 98 by means of the thumb screws 214 and 216.The guide members 210 and 212 are of relatively large diameter, so as toprovide the guide edges 218 and 220 which act as a second positivelateral reference point for the chart web 24 as it is being withdrawnfrom the chart pack 16.

in accordance with a further aspect of the invention, a reference wallor fence is provided for the chart pack 16 by means of a bracketindicated generally at-222 which may be adjustably positioned on theshelf 18 by means of the thumbscrews 224 and 226 which extend throughslots 228 in the rear flange 230 of the bracket 222. The bracket 222includes an end wall 232 against which the rear edge of the chart pack16 may be positioned when the chart pack is loaded into the recorder,the position of the rear wall 232 providing the basic reference locationfor the inner edge of the chart web '24 relative to the upper elementsof the transport mechanism. The end walls 234 and 236 of the bracket 222are provided for approximate location of the chart pack in thisdimension and are spaced apart an amount sufficient to provide adequateclearance to permit the chart pack 16 to be referenced against the endwall 232.

After the chart pack 16 has been moved into engagement with the rearwall 232 the actuating crank 71 is moved to the position shown in dottedlines in FIG. 1 in which position the above-described overcenter togglelinkages are actuated to permit insertion of the chart lead between theguide bar 104 and the back tensioning roller 90 and upwardly over thetilted writing table 26. When the chart lead is threaded through thetransport mechanism the crank 71 is moved back to the normal operatingposition and the guide members 200 and 210 are positioned on the guidebars 104 and 98, respectively, so that the guide edges 206 and 218 arepositioned at the correct point to control the outer edge of the chartweb 24. The guide members 202 and 212 are initially positioned inalignment with the reference wall 232 of the bracket 222 and remain inthis adjusted position irrespective of the width of the chart paperwhich is used.

Since the guide bar 104 must be held in close proximity to theback-tension roller 90 in order to prevent the chart from slippingrelative to this roller during changes in chart speed and at high chartspeeds, the guide flanges 206 and 208 are necessarily of relativelylimited height. The Z-fold chart will not readily follow the roundedcontours of the guide bars 104 and 98, especially at the folds of thechart-paper, and if only the guide flanges 206 and 208 were provided,the chart paper might escape over these edges thus resulting in a lossof lateral reference. In accordance with a further feature of theinvention, the leaf springs 102 provide an additional small amount ofload on the chart web 24 at the guide bar 98 and produce a sufficientamount of friction between the chart web 24 and the' guide bar 98 tohold the chart in conformity to the contour of guide bar 104 and thuswithin thecontrol of the edge guides 206 and 208. The edge guide 206,208'and 218, 220 thus act as four spaced-apart reference points whichestablish a positive lateral position for the chart web even thoughZ-fold chart paper is used. Accordingly, the nonskew drive rollers 28will produce an essentially zero-weave delivery of the chart.

In accordance with a further aspect of the invention, different widthsof chart paper may be used while requiring a minimum number ofadjustments by the operator and without requiring readjustment of thebacktension roller 90 or the drive rollers 28. More particularly, it hasbeen found that by using only three drive roller cartridges 28 properlyspaced across the width of the transport, zero-weave delivery of chartsof several different widths may be provided without changing the driveroller locations. One of the three drive roller cartridges is placed soas to be centered on the eight channel, i.e., maximum width chart, asshown in FIG. 1. The other two drive roller cartridges are placed at theouter edges of the first and last channels of eight-channel-width chartpaper. For eight-channel chart paper, weave is controlled by the twoouter rollers 28, the center roller having relatively small effect underthese conditions. For fourand six-channelwidth charts, the outermost oneof the drive roller cartridges, i.e., the one closest to the sidewall 33(FIG.'1) does not engage the chart since the inner chart edge isreferenced against the locating fence 232 for all widths of chart paper,and is used as a constant reference position for the inner edge of allcharts. The two engaged rollers, i.e., the center roller and theinnermost roller, while not exactly symmetrical on the width of thechart, are sufficiently off center to effect a zero-weave delivery ofboth four-channeland six-channel-width charts. For two-channel-widthcharts, only the innermost roller 28 engages the-chart paper but thenarrow width of the paper allows the edge guides 206, 208 and 218, 220to control the chart position with essentially zero-weave delivery.

When the width of the chart paper is changed to either sixchannel chartpaper or four-channel chart paper the guide members 200 and 210 aremoved along the length of the guide bars 104 and 98, respectively, sothat the guide edges 208 and 216 contact the edge of the new width ofchart paper used in the recorder after which the setscrews 204 and 214are tightened. These two adjustments are the only ones required to adaptthe transport mechanism to a different width of chart paper, the members202 and 212 remaining in their originally adjusted positions which arein alignment with the reference fence 232.

In order to eliminate the need for readjustment of the backtensionroller and readjustment of the slip torque of the drive rollercartridges 28 when different widths of chart paper are used, the leafsprings 102 are provided in the form of four independent spring membersdifferent numbers of which are arranged to contact the chart paperdepending upon the width of chart paper which-is being used. Moreparticularly, four independent leaf springs are secured to the crossbar.35. The first of these leaf springs 102a (FIG. 3) is positioned toengage the chart paper between the channel 1 portion of eight-channelchart paper, which is identified as the area 240 in FIG. 3, and thesecond channel area 242.

The second leaf spring (not shown) is arranged to engage the chart paperbetween the third and fourth channels of eight-channel-width chartpaper. The third leaf spring (not shown) is arranged to engage the chartpaper between the fifth and sixth channel areas of the chart and thefourth leaf spring 102d (FIG. 3) is arranged to contact the chart paperbetween the seventh channel area 244 and the eighth channel area 246.With this arrangement all four leaf springs 102 will contact eightchannel width chart paper, only three of these springs will contactsix-channel chart paper, only two will contact four-channel chart paper,and only the leaf spring 102a will contact two channel width chartpaper. The setting of the back-tension roller'90 and the load rate forthe leaf springs 102 is determined by testing with each width of chartpaper so that the web tension provided by the leaf springs 102, for eachwidth of chart, added to the tension provided by the back-tension roller90, is sufficient to provide zero-weave delivery of each width of chartwith no adjustment of the back-tension roller 90 or readjustment of theslip torque of the drive roller cartridges 28.

The user of the recorder need change only the outer two guide members200 and 210, as described heretofore, in changing from one width ofchart to another. In this connection, it is pointed out that duringinitial loading of a chart pack of any width into the recorder, thecrank 71 is actuated to the loading position in which position the leafsprings 102 are spaced from the guide bar 98 and the back tensioningroller 90 to permit easy loading of the chart paper into the transportmechanism with any width of chart, as described in more detail in theabove-identified copending application of Albert Kupferschmidt.

While but a single embodiment of the present invention has been herespecifically disclosed, it will be apparent that many variations may bemade therein, all within the true spirit and scope of the invention, asdefined in the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a chart recorder, the combination of, a supply of Z-fold chartpaper, a back-tension roller system including a back-tension roller anda guide roller for holding the chart paper against the outer surface ofsaid back-tension roller and for controlling the point at which thechart paper leaves said outer surface, a writing table positioned abovesaid back-tension roller system, means for positioning said chart supplyrelative to said back-tension roller system, a plurality of driverollers and mating pressure rollers positioned at the forward end ofsaid writing table and arranged to pull the chart paper over saidwriting table against the force of said back-tension roller system, saiddrive rollers being arranged individually to slip when a predeterminedtorque is exerted thereon so that a nonskew drive system for the chartpaper is provided, and spacedapart guide means positioned between saidback-tension roller and said chart supply to provide a positive lateralreference for the chart paper prior to its engagement with saidback-tension roller, said guide means being adjustable to accommodatechart paper of different widths and to provide the proper positivelateral reference for each width of chart paper used.

2. The combination of claim 1, wherein said guide means includes a firstguide bar in close proximity to said back tensioning roller and havingguide flanges on the ends thereof which are adapted to guide the edgesof the chart paper as it is engaged by said back tensioning roller.

3. The combination of claim 2 wherein a second guide bar is positionedbetween said first guide bar and said chart supply, said second guidebar having guide flanges on the ends thereof which are also adapted toguide the edges of the chart paper.

4. The combination of claim 2, wherein one of said guide flanges on saidfirst guide bar is adjustable along the length thereof to accommodatechart paper of different widths.

5. The combination of claim 3, wherein one of said guide flanges on saidsecond guide bar is adjustable along the length thereof to accommodatechart paper of different widths.

6. The combination of claim 3, wherein leaf spring means is positionedto engage the chart paper passing over said second guide bar to hold theedges of the chart paper in engagement with said guide flanges'on saidfirst guide bar.

7. The combination of claim 1, wherein said plurality of drive rollerscomprise three drive rollers one of which is positioned to engage chartpaper of maximum width at the center thereof and the other two of whichare positioned to engage the edges of chart paper of maximum width.

8. The combination of claim 7, wherein said guide means includes a guidebar-over which paper is drawn from said chart supply by said driverollers, and chart paper tension means adapted to engage the chart paperas it is drawn over said guide bar, said chart paper tension means beingadapted to engage chart paper of different widths.

9,. The combination of claim 8, wherein said chart paper tension meanscomprises a plurality of leaf spring fingers spaced along the length ofsaid guide bar so that chart paper of different widths is engaged bydifferent members of said spring fingers.

1. In a chart recorder, the combination of, a supply of Z-fold chartpaper, a back-tension roller system including a backtension roller and aguide roller for holding the chart paper against the outer surface ofsaid back-tension roller and for controlling the point at which thechart paper leaves said outer surface, a writing table positioned abovesaid back-tension roller system, means for positioning said chart supplyrelative to said back-tension roller system, a plurality of driverollers and mating pressure rollers positioned at the forward end ofsaid writing table and arranged to pull the chart paper over saidwriting table against the force of said back-tension roller system, saiddrive rollers being arranged individually to slip when a predeterminedtorque is exerted thereon so that a nonskew drive system for the chartpaper is provided, and spaced-apart guide means positioned between saidback-tension roller and said chart supply to provide a positive lateralreference for the chart paper prior to its engagement with saidback-tension roller, said guide means being adjustable to accommodatechart paper of different widths and to provide the proper positivelateral reference for each width of chart paper used.
 2. The combinationof claim 1, wherein said guide means includes a first guide bar in closeproximity to said back tensioning roller and having guide flanges on theends thereof which are adapted to guide the edges of the chart paper asit is engaged by said back tensioning roller.
 3. The combination ofclaim 2 wherein a second guide bar is positioned between said firstguide bar aNd said chart supply, said second guide bar having guideflanges on the ends thereof which are also adapted to guide the edges ofthe chart paper.
 4. The combination of claim 2, wherein one of saidguide flanges on said first guide bar is adjustable along the lengththereof to accommodate chart paper of different widths.
 5. Thecombination of claim 3, wherein one of said guide flanges on said secondguide bar is adjustable along the length thereof to accommodate chartpaper of different widths.
 6. The combination of claim 3, wherein leafspring means is positioned to engage the chart paper passing over saidsecond guide bar to hold the edges of the chart paper in engagement withsaid guide flanges on said first guide bar.
 7. The combination of claim1, wherein said plurality of drive rollers comprise three drive rollersone of which is positioned to engage chart paper of maximum width at thecenter thereof and the other two of which are positioned to engage theedges of chart paper of maximum width.
 8. The combination of claim 7,wherein said guide means includes a guide bar over which paper is drawnfrom said chart supply by said drive rollers, and chart paper tensionmeans adapted to engage the chart paper as it is drawn over said guidebar, said chart paper tension means being adapted to engage chart paperof different widths.
 9. The combination of claim 8, wherein said chartpaper tension means comprises a plurality of leaf spring fingers spacedalong the length of said guide bar so that chart paper of differentwidths is engaged by different members of said spring fingers.